]> Ericsson

Sweden goran.selander@ericsson.com

Ericsson

Sweden john.mattsson@ericsson.com

ASSA ABLOY

Poland marek.serafin@assaabloy.com

RISE

Sweden marco.tiloca@ri.se

Inria

France malisa.vucinic@inria.fr

Security LAKE Working Group Internet-Draft This document contains some example traces of Ephemeral Diffie-Hellman Over COSE (EDHOC).

Introduction EDHOC is a lightweight authenticated key exchange protocol designed for highly constrained settings. This document contains annotated traces of EDHOC protocol runs, with input, output, and intermediate processing results to simplify testing of implementations. The document contains two traces:

• - Authentication with signature keys identified by the hash value of the X.509 certificates (provided in ). The endpoints use EdDSA for authentication and X25519 for ephemeral-ephemeral Diffie-Hellman key exchange.
• - Authentication with static Diffie-Hellman keys identified by short key identifiers labelling CWT Claim Sets (CCSs) . The endpoints use NIST P-256 for both ephemeral-ephemeral and static-ephemeral Diffie-Hellman key exchange. This trace also illustrates the cipher suite negotiation, and provides an example of low protocol overhead, with messages sizes of (39, 45, 19) bytes.

The traces in this draft are valid for version -22 of . The traces has been verified by two independent implementations.

Setup EDHOC is run between an Initiator (I) and a Responder (R). The private/public key pairs and credentials of the Initiator and the Responder required to produce the protocol messages are shown in the traces when needed for the calculations. EDHOC messages and intermediate results are encoded in CBOR and can therefore be displayed in CBOR diagnostic notation using, e.g., the CBOR playground , which makes them easy to parse for humans. NOTE 1. The same name is used for hexadecimal byte strings and their CBOR encodings. The traces contain both the raw byte strings and the corresponding CBOR encoded data items. NOTE 2. If not clear from the context, remember that CBOR sequences and CBOR arrays assume CBOR encoded data items as elements. NOTE 3. When the protocol transporting EDHOC messages does not inherently provide correlation across all messages, like CoAP , then some messages typically are prepended with connection identifiers and potentially a message_1 indicator (see Sections and of ). Those bytes are not included in the traces in this document.

Authentication with Signatures, X.509 Certificates Identified by 'x5t' In this example the Initiator (I) and Responder (R) are authenticated with digital signatures (METHOD = 0). Both the Initiator and the Responder support cipher suite 0, which determines the algorithms:

• EDHOC AEAD algorithm = AES-CCM-16-64-128
• EDHOC hash algorithm = SHA-256
• EDHOC MAC length in bytes (Static DH) = 8
• EDHOC key exchange algorithm (ECDH curve) = X25519
• EDHOC signature algorithm = EdDSA
• Application AEAD algorithm = AES-CCM-16-64-128
• Application hash algorithm = SHA-256

The public keys are represented with X.509 certificates identified by the COSE header parameter 'x5t'.

message_1 Both endpoints are authenticated with signatures, i.e., METHOD = 0: The Initiator selects cipher suite 0. A single cipher suite is encoded as an int: The Initiator creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Initiator selects its connection identifier C_I to be the byte string 0x2d, which since it is represented by the 1-byte CBOR int -14 is encoded as 0x2d: No external authorization data: The Initiator constructs message_1:

message_2 The Responder supports the most preferred and selected cipher suite 0, so SUITES_I is acceptable. The Responder creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Responder selects its connection identifier C_R to be the byte string 0x18, which since it is not represented as a 1-byte CBOR int is encoded as h'18' = 0x4118: The transcript hash TH_2 is calculated using the EDHOC hash algorithm: TH_2 = H( G_Y, H(message_1) ) The input to calculate TH_2 is the CBOR sequence: G_Y, H(message_1) PRK_2e is specified in . First, the ECDH shared secret G_XY is computed from G_X and Y, or G_Y and X: Then, PRK_2e is calculated using EDHOC_Extract() determined by the EDHOC hash algorithm: where salt is TH_2: Since METHOD = 0, the Responder authenticates using signatures. Since the selected cipher suite is 0, the EDHOC signature algorithm is EdDSA. The Responder's signature key pair using EdDSA: PRK_3e2m is specified in . Since the Responder authenticates with signatures PRK_3e2m = PRK_2e. The Responder constructs the remaining input needed to calculate MAC_2: MAC_2 = EDHOC_KDF( PRK_3e2m, 2, context_2, mac_length_2 ) context_2 = << ID_CRED_R, TH_2, CRED_R, ? EAD_2 >> CRED_R is identified by a 64-bit hash: where the COSE header value 34 ('x5t') indicates a hash of an X.509 certficate, and the COSE algorithm -15 indicates the hash algorithm SHA-256 truncated to 64 bits. CRED_R is a CBOR byte string of the DER encoding of the X.509 certificate in : No external authorization data: context_2 = << ID_CRED_R, TH_2, CRED_R, ? EAD_2 >> MAC_2 is computed through EDHOC_Expand() using the EDHOC hash algorithm, see : MAC_2 = HKDF-Expand(PRK_3e2m, info, mac_length_2), where info = ( 2, context_2, mac_length_2 ) Since METHOD = 0, mac_length_2 is given by the EDHOC hash algorithm. info for MAC_2 is: where the last value is the output size of the EDHOC hash algorithm in bytes. Since METHOD = 0, Signature_or_MAC_2 is the 'signature' of the COSE_Sign1 object. The Responder constructs the message to be signed: >, << TH_2, CRED_R, ? EAD_2 >>, MAC_2 ] = [ "Signature1", h'a11822822e4879f2a41b510c1f9b', h'5820c6405c154c567466ab1df20369500e540e9f14bd3a79 6a0652cae66c9061688d58f13081ee3081a1a00302010202 0462319ec4300506032b6570301d311b301906035504030c 124544484f4320526f6f742045643235353139301e170d32 32303331363038323433365a170d32393132333132333030 30305a30223120301e06035504030c174544484f43205265 73706f6e6465722045643235353139302a300506032b6570 032100a1db47b95184854ad12a0c1a354e418aace33aa0f2 c662c00b3ac55de92f9359300506032b6570034100b723bc 01eab0928e8b2b6c98de19cc3823d46e7d6987b032478fec faf14537a1af14cc8be829c6b73044101837eb4abc949565 d86dce51cfae52ab82c152cb02', h'369ca4392c83ed63d61ad218420ea36706008478d5bc3049 fb8c5942444b1333' ] ]]> The Responder signs using the private authentication key SK_R The Responder constructs PLAINTEXT_2: The input needed to calculate KEYSTREAM_2 is defined in , using EDHOC_Expand() with the EDHOC hash algorithm: where plaintext_length is the length in bytes of PLAINTEXT_2 in bytes, and info for KEYSTREAM_2 is: where the last value is the length in bytes of PLAINTEXT_2. The Responder calculates CIPHERTEXT_2 as XOR between PLAINTEXT_2 and KEYSTREAM_2: The Responder constructs message_2: where G_Y_CIPHERTEXT_2 is the bstr encoding of the concatenation of the raw values of G_Y and CIPHERTEXT_2.

message_3 Since METHOD = 0, the Initiator authenticates using signatures. Since the selected cipher suite is 0, the EDHOC signature algorithm is EdDSA. The Initiator's signature key pair using EdDSA: PRK_4e3m is specified in . Since the Initiator authenticates with signatures PRK_4e3m = PRK_3e2m. The transcript hash TH_3 is calculated using the EDHOC hash algorithm: TH_3 = H(TH_2, PLAINTEXT_2, CRED_R) The Initiator constructs the remaining input needed to calculate MAC_3: where > ]]> CRED_I is identified by a 64-bit hash: where the COSE header value 34 ('x5t') indicates a hash of an X.509 certficate, and the COSE algorithm -15 indicates the hash algorithm SHA-256 truncated to 64 bits. CRED_I is a CBOR byte string of the DER encoding of the X.509 certificate in : No external authorization data: context_3 = << ID_CRED_I, TH_3, CRED_I, ? EAD_3 >> MAC_3 is computed through EDHOC_Expand() using the EDHOC hash algorithm, see : info = ( 6, context_3, mac_length_3 ) where context_3 = << ID_CRED_I, TH_3, CRED_I, ? EAD_3 >> Since METHOD = 0, mac_length_3 is given by the EDHOC hash algorithm. info for MAC_3 is: where the last value is the output size of the EDHOC hash algorithm in bytes. Since METHOD = 0, Signature_or_MAC_3 is the 'signature' of the COSE_Sign1 object. The Initiator constructs the message to be signed: >, << TH_3, CRED_I, ? EAD_3 >>, MAC_3 ] = [ "Signature1", h'a11822822e48c24ab2fd7643c79f', h'5820e091121af5ac6ce2145d4825e09012f29798e8f713ac 9891432d2256b6f678e958f13081ee3081a1a00302010202 0462319ea0300506032b6570301d311b301906035504030c 124544484f4320526f6f742045643235353139301e170d32 32303331363038323430305a170d32393132333132333030 30305a30223120301e06035504030c174544484f4320496e 69746961746f722045643235353139302a300506032b6570 032100ed06a8ae61a829ba5fa54525c9d07f48dd44a302f4 3e0f23d8cc20b73085141e300506032b6570034100521241 d8b3a770996bcfc9b9ead4e7e0a1c0db353a3bdf2910b392 75ae48b756015981850d27db6734e37f67212267dd05eeff 27b9e7a813fa574b72a00b430b', h'51c968a7f9fdea19c7023f7022b4d9f214772ef588590524 0576f62d036e69dc' ] ]]> The Initiator signs using the private authentication key SK_I: The Initiator constructs PLAINTEXT_3: The Initiator constructs the associated data for message_3: The Initiator constructs the input needed to derive the key K_3, see , using the EDHOC hash algorithm: where key_length is the key length in bytes for the EDHOC AEAD algorithm, and info for K_3 is: where the last value is the key length in bytes for the EDHOC AEAD algorithm. The Initiator constructs the input needed to derive the nonce IV_3, see , using the EDHOC hash algorithm: where iv_length is the nonce length in bytes for the EDHOC AEAD algorithm, and info for IV_3 is: where the last value is the nonce length in bytes for the EDHOC AEAD algorithm. The Initiator calculates CIPHERTEXT_3 as 'ciphertext' of COSE_Encrypt0 applied using the EDHOC AEAD algorithm with plaintext PLAINTEXT_3, additional data A_3, key K_3 and nonce IV_3. message_3 is the CBOR bstr encoding of CIPHERTEXT_3: The transcript hash TH_4 is calculated using the EDHOC hash algorithm: TH_4 = H( TH_3, PLAINTEXT_3, CRED_I )

message_4 No external authorization data: The Responder constructs PLAINTEXT_4: The Responder constructs the associated data for message_4: The Responder constructs the input needed to derive the EDHOC message_4 key, see , using the EDHOC hash algorithm: where key_length is the key length in bytes for the EDHOC AEAD algorithm, and info for K_4 is: where the last value is the key length in bytes for the EDHOC AEAD algorithm. The Responder constructs the input needed to derive the EDHOC message_4 nonce, see , using the EDHOC hash algorithm: where length is the nonce length in bytes for the EDHOC AEAD algorithm, and info for IV_4 is: where the last value is the nonce length in bytes for the EDHOC AEAD algorithm. The Responder calculates CIPHERTEXT_4 as 'ciphertext' of COSE_Encrypt0 applied using the EDHOC AEAD algorithm with plaintext PLAINTEXT_4, additional data A_4, key K_4 and nonce IV_4. message_4 is the CBOR bstr encoding of CIPHERTEXT_4:

PRK_out and PRK_exporter PRK_out is specified in . where hash_length is the length in bytes of the output of the EDHOC hash algorithm, and info for PRK_out is: where the last value is the length in bytes of the output of the EDHOC hash algorithm. The OSCORE Master Secret and OSCORE Master Salt are derived with the EDHOC_Exporter as specified in . where PRK_exporter is derived from PRK_out: where hash_length is the length in bytes of the output of the EDHOC hash algorithm, and info for the PRK_exporter is: where the last value is the length in bytes of the output of the EDHOC hash algorithm.

OSCORE Parameters The derivation of OSCORE parameters is specified in . The AEAD and Hash algorithms to use in OSCORE are given by the selected cipher suite: The mapping from EDHOC connection identifiers to OSCORE Sender/Recipient IDs is defined in . C_R is mapped to the Recipient ID of the server, i.e., the Sender ID of the client. The byte string 0x18, which as C_R is encoded as the CBOR byte string 0x4118, is converted to the server Recipient ID 0x18. C_I is mapped to the Recipient ID of the client, i.e., the Sender ID of the server. The byte string 0x2d, which as C_I is encoded as the CBOR integer 0x2d is converted to the client Recipient ID 0x2d. The OSCORE Master Secret is computed through EDHOC_Expand() using the Application hash algorithm, see : where oscore_key_length is by default the key length in bytes for the Application AEAD algorithm, and info for the OSCORE Master Secret is: where the last value is the key length in bytes for the Application AEAD algorithm. The OSCORE Master Salt is computed through EDHOC_Expand() using the Application hash algorithm, see : where oscore_salt_length is the length in bytes of the OSCORE Master Salt, and info for the OSCORE Master Salt is: where the last value is the length in bytes of the OSCORE Master Salt.

Key Update Key update is defined in . where hash_length is the length in bytes of the output of the EDHOC hash function, and context for KeyUpdate is and where info for key update is: After key update, the PRK_exporter needs to be derived anew: where info and hash_length are unchanged as in . The OSCORE Master Secret is derived with the updated PRK_exporter: where info and key_length are unchanged as in . The OSCORE Master Salt is derived with the updated PRK_exporter: where info and salt_length are unchanged as in .

Certificates

Responder Certificate

Initiator Certificate

Common Root Certificate

Authentication with Static DH, CCS Identified by 'kid' In this example the Initiator and the Responder are authenticated with ephemeral-static Diffie-Hellman (METHOD = 3). The Initiator supports cipher suites 6 and 2 (in order of preference) and the Responder only supports cipher suite 2. After an initial negotiation message exchange, cipher suite 2 is used, which determines the algorithms:

• EDHOC AEAD algorithm = AES-CCM-16-64-128
• EDHOC hash algorithm = SHA-256
• EDHOC MAC length in bytes (Static DH) = 8
• EDHOC key exchange algorithm (ECDH curve) = P-256
• EDHOC signature algorithm = ES256
• Application AEAD algorithm = AES-CCM-16-64-128
• Application hash algorithm = SHA-256

The public keys are represented as raw public keys (RPK), encoded in a CWT Claims Set (CCS) and identified by the COSE header parameter 'kid'.

message_1 (first time) Both endpoints are authenticated with static DH, i.e., METHOD = 3: The Initiator selects its preferred cipher suite 6. A single cipher suite is encoded as an int: The Initiator creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Initiator selects its connection identifier C_I to be the byte string 0x0e, which since it is represented by the 1-byte CBOR int 14 is encoded as 0x0e: No external authorization data: EAD_1 (CBOR Sequence) (0 bytes) The Initiator constructs message_1:

error The Responder does not support cipher suite 6 and sends an error with ERR_CODE 2 containing SUITES_R as ERR_INFO. The Responder proposes cipher suite 2, a single cipher suite thus encoded as an int.

message_1 (second time) Same steps are performed as for message_1 the first time, , but with updated SUITES_I. Both endpoints are authenticated with static DH, i.e., METHOD = 3: The Initiator selects cipher suite 2 and indicates the more preferred cipher suite(s), in this case 6, all encoded as the array [6, 2]: The Initiator creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Initiator selects its connection identifier C_I to be the byte string 0x37, which since it is represented by the 1-byte CBOR int -24 is encoded as 0x37: No external authorization data: The Initiator constructs message_1:

message_2 The Responder supports the selected cipher suite 2 and not the by the Initiator more preferred cipher suite(s) 6, so SUITES_I is acceptable. The Responder creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Responder selects its connection identifier C_R to be the byte string 0x27, which since it is represented by the 1-byte CBOR int -8 is encoded as 0x27: The transcript hash TH_2 is calculated using the EDHOC hash algorithm: TH_2 = H( G_Y, H(message_1) ) The input to calculate TH_2 is the CBOR sequence: G_Y, H(message_1) PRK_2e is specified in . First, the ECDH shared secret G_XY is computed from G_X and Y, or G_Y and X: Then, PRK_2e is calculated using EDHOC_Extract() determined by the EDHOC hash algorithm: where salt is TH_2: Since METHOD = 3, the Responder authenticates using static DH. The EDHOC key exchange algorithm is based on the same curve as for the ephemeral keys, which is P-256, since the selected cipher suite is 2. The Responder's static Diffie-Hellman P-256 key pair: Since the Responder authenticates with static DH (METHOD = 3), PRK_3e2m is derived from SALT_3e2m and G_RX. The input needed to calculate SALT_3e2m is defined in , using EDHOC_Expand() with the EDHOC hash algorithm: where hash_length is the length in bytes of the output of the EDHOC hash algorithm, and info for SALT_3e2m is: PRK_3e2m is specified in . PRK_3e2m is derived from G_RX using EDHOC_Extract() with the EDHOC hash algorithm: where G_RX is the ECDH shared secret calculated from G_X and R, or G_R and X. The Responder constructs the remaining input needed to calculate MAC_2: MAC_2 = EDHOC_KDF( PRK_3e2m, 2, context_2, mac_length_2 ) context_2 = << ID_CRED_R, TH_2, CRED_R, ? EAD_2 >> CRED_R is identified by a 'kid' with byte string value 0x32: CRED_R is an RPK encoded as a CCS: No external authorization data: context_2 = << ID_CRED_R, TH_2, CRED_R, ? EAD_2 >> MAC_2 is computed through EDHOC_Expand() using the EDHOC hash algorithm, see : MAC_2 = HKDF-Expand(PRK_3e2m, info, mac_length_2), where info = ( 2, context_2, mac_length_2 ) Since METHOD = 3, mac_length_2 is given by the EDHOC MAC length. info for MAC_2 is: where the last value is the EDHOC MAC length in bytes. Since METHOD = 3, Signature_or_MAC_2 is MAC_2: The Responder constructs PLAINTEXT_2: Since ID_CRED_R contains a single 'kid' parameter, only the byte string value is included in the plaintext, represented as described in . The CBOR map { 4 : h'32' } is thus replaced, not by the CBOR byte string 0x4132, but by the CBOR int 0x32, since that is a one byte encoding of a CBOR integer (-19). The input needed to calculate KEYSTREAM_2 is defined in , using EDHOC_Expand() with the EDHOC hash algorithm: where plaintext_length is the length in bytes of PLAINTEXT_2, and info for KEYSTREAM_2 is: where the last value is the length in bytes of PLAINTEXT_2. The Responder calculates CIPHERTEXT_2 as XOR between PLAINTEXT_2 and KEYSTREAM_2: The Responder constructs message_2: where G_Y_CIPHERTEXT_2 is the bstr encoding of the concatenation of the raw values of G_Y and CIPHERTEXT_2.

message_3 The transcript hash TH_3 is calculated using the EDHOC hash algorithm: TH_3 = H( TH_2, PLAINTEXT_2, CRED_R ) Since METHOD = 3, the Initiator authenticates using static DH. The EDHOC key exchange algorithm is based on the same curve as for the ephemeral keys, which is P-256, since the selected cipher suite is 2. The Initiator's static Diffie-Hellman P-256 key pair: Since I authenticates with static DH (METHOD = 3), PRK_4e3m is derived from SALT_4e3m and G_IY. The input needed to calculate SALT_4e3m is defined in , using EDHOC_Expand() with the EDHOC hash algorithm: where hash_length is the length in bytes of the output of the EDHOC hash algorithm, and info for SALT_4e3m is: PRK_4e3m is specified in . Since I authenticates with static DH (METHOD = 3), PRK_4e3m is derived from G_IY using EDHOC_Extract() with the EDHOC hash algorithm: where G_IY is the ECDH shared secret calculated from G_I and Y, or G_Y and I. The Initiator constructs the remaining input needed to calculate MAC_3: MAC_3 = EDHOC_KDF( PRK_4e3m, 6, context_3, mac_length_3 ) context_3 = << ID_CRED_I, TH_3, CRED_I, ? EAD_3 >> CRED_I is identified by a 'kid' with byte string value 0x2b: CRED_I is an RPK encoded as a CCS: No external authorization data: context_3 = << ID_CRED_I, TH_3, CRED_I, ? EAD_3 >> MAC_3 is computed through EDHOC_Expand() using the EDHOC hash algorithm, see : info = ( 6, context_3, mac_length_3 ) Since METHOD = 3, mac_length_3 is given by the EDHOC MAC length. info for MAC_3 is: where the last value is the EDHOC MAC length in bytes. Since METHOD = 3, Signature_or_MAC_3 is MAC_3: The Initiator constructs PLAINTEXT_3: Since ID_CRED_I contains a single 'kid' parameter, only the byte string value is included in the plaintext, represented as described in . The CBOR map { 4 : h'2b' } is thus replaced, not by the CBOR byte string 0x412b, but by the CBOR int 0x2b, since that is a one byte encoding of a CBOR integer (-12). The Initiator constructs the associated data for message_3: The Initiator constructs the input needed to derive the key K_3, see , using the EDHOC hash algorithm: where key_length is the key length in bytes for the EDHOC AEAD algorithm, and info for K_3 is: where the last value is the key length in bytes for the EDHOC AEAD algorithm. The Initiator constructs the input needed to derive the nonce IV_3, see , using the EDHOC hash algorithm: where iv_length is the nonce length in bytes for the EDHOC AEAD algorithm, and info for IV_3 is: where the last value is the nonce length in bytes for the EDHOC AEAD algorithm. The Initiator calculates CIPHERTEXT_3 as 'ciphertext' of COSE_Encrypt0 applied using the EDHOC AEAD algorithm with plaintext PLAINTEXT_3, additional data A_3, key K_3 and nonce IV_3. message_3 is the CBOR bstr encoding of CIPHERTEXT_3: The transcript hash TH_4 is calculated using the EDHOC hash algorithm: TH_4 = H( TH_3, PLAINTEXT_3, CRED_I )

message_4 No external authorization data: EAD_4 (CBOR Sequence) (0 bytes) The Responder constructs PLAINTEXT_4: PLAINTEXT_4 (CBOR Sequence) (0 bytes) The Responder constructs the associated data for message_4: The Responder constructs the input needed to derive the EDHOC message_4 key, see , using the EDHOC hash algorithm: where key_length is the key length in bytes for the EDHOC AEAD algorithm, and info for K_4 is: where the last value is the key length in bytes for the EDHOC AEAD algorithm. The Responder constructs the input needed to derive the EDHOC message_4 nonce, see , using the EDHOC hash algorithm: where iv_length is the nonce length in bytes for the EDHOC AEAD algorithm, and info for IV_4 is: where the last value is the nonce length in bytes for the EDHOC AEAD algorithm. The Responder calculates CIPHERTEXT_4 as 'ciphertext' of COSE_Encrypt0 applied using the EDHOC AEAD algorithm with plaintext PLAINTEXT_4, additional data A_4, key K_4 and nonce IV_4. message_4 is the CBOR bstr encoding of CIPHERTEXT_4:

PRK_out and PRK_exporter PRK_out is specified in . where hash_length is the length in bytes of the output of the EDHOC hash algorithm, and info for PRK_out is: where the last value is the length in bytes of the output of the EDHOC hash algorithm. The OSCORE Master Secret and OSCORE Master Salt are derived with the EDHOC_Exporter as specified in 4.2.1 of . where PRK_exporter is derived from PRK_out: where hash_length is the length in bytes of the output of the EDHOC hash algorithm, and info for the PRK_exporter is: where the last value is the length in bytes of the output of the EDHOC hash algorithm.

OSCORE Parameters The derivation of OSCORE parameters is specified in . The AEAD and Hash algorithms to use in OSCORE are given by the selected cipher suite: The mapping from EDHOC connection identifiers to OSCORE Sender/Recipient IDs is defined in . C_R is mapped to the Recipient ID of the server, i.e., the Sender ID of the client. The byte string 0x27, which as C_R is encoded as the CBOR integer 0x27, is converted to the server Recipient ID 0x27. C_I is mapped to the Recipient ID of the client, i.e., the Sender ID of the server. The byte string 0x37, which as C_I is encoded as the CBOR integer 0x0e is converted to the client Recipient ID 0x37. The OSCORE Master Secret is computed through EDHOC_Expand() using the Application hash algorithm, see : where oscore_key_length is by default the key length in bytes for the Application AEAD algorithm, and info for the OSCORE Master Secret is: where the last value is the key length in bytes for the Application AEAD algorithm. The OSCORE Master Salt is computed through EDHOC_Expand() using the Application hash algorithm, see : where oscore_salt_length is the length in bytes of the OSCORE Master Salt, and info for the OSCORE Master Salt is: where the last value is the length in bytes of the OSCORE Master Salt.

Key Update Key update is defined in . where hash_length is the length in bytes of the output of the EDHOC hash function, context for KeyUpdate is and where info for key update is: After key update the PRK_exporter needs to be derived anew: where info and hash_length are unchanged as in . The OSCORE Master Secret is derived with the updated PRK_exporter: where info and key_length are unchanged as in . The OSCORE Master Salt is derived with the updated PRK_exporter: where info and salt_length are unchanged as in .

Security Considerations This document contains examples of EDHOC whose security considerations apply. The keys printed in these examples cannot be considered secret and must not be used.

IANA Considerations There are no IANA considerations.

Informative References Ericsson AB Ericsson AB Ericsson AB This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios and a main use case is to establish an OSCORE security context. By reusing COSE for cryptography, CBOR for encoding, and CoAP for transport, the additional code size can be kept very low. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. This memo specifies two elliptic curves over prime fields that offer a high level of practical security in cryptographic applications, including Transport Layer Security (TLS). These curves are intended to operate at the ~128-bit and ~224-bit security level, respectively, and are generated deterministically based on a list of required properties. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format.

Acknowledgments The authors want to thank all people verifying EDHOC test vectors and/or contributing to the interoperability testing including: , , , , , , , , and .