Crypto
Hash algorithmsâ€‹
The builtin enumeration HashAlgorithm
provides the set of supported hashing algorithms.
_41access(all)_41enum HashAlgorithm: UInt8 {_41 /// SHA2_256 is SHA2 with a 256bit digest (also referred to as SHA256)._41 access(all)_41 case SHA2_256 = 1_41_41 /// SHA2_384 is SHA2 with a 384bit digest (also referred to as SHA384)._41 access(all)_41 case SHA2_384 = 2_41_41 /// SHA3_256 is SHA3 with a 256bit digest._41 access(all)_41 case SHA3_256 = 3_41_41 /// SHA3_384 is SHA3 with a 384bit digest._41 access(all)_41 case SHA3_384 = 4_41_41 /// KMAC128_BLS_BLS12_381 is an instance of KECCAK Message Authentication Code (KMAC128) mac algorithm._41 /// Although this is a MAC algorithm, KMAC is included in this list as it can be used hash_41 /// when the key is used a nonpublic customizer._41 /// KMAC128_BLS_BLS12_381 is used in particular as the hashing algorithm for the BLS signature scheme on the curve BLS12381._41 /// It is a customized version of KMAC128 that is compatible with the hashing to curve_41 /// used in BLS signatures._41 /// It is the same hasher used by signatures in the internal Flow protocol._41 access(all)_41 case KMAC128_BLS_BLS12_381 = 5_41_41 /// KECCAK_256 is the legacy Keccak algorithm with a 256bits digest, as per the original submission to the NIST SHA3 competition._41 /// KECCAK_256 is different than SHA3 and is used by Ethereum._41 access(all)_41 case KECCAK_256 = 6_41_41 /// Returns the hash of the given data_41 access(all)_41 view fun hash(_ data: [UInt8]): [UInt8]_41_41 /// Returns the hash of the given data and tag_41 access(all)_41 view fun hashWithTag(_ data: [UInt8], tag: string): [UInt8]_41}
The hash algorithms provide two ways to hash input data into digests, hash
and hashWithTag
.
Hashingâ€‹
hash
hashes the input data using the chosen hashing algorithm.
KMAC
is the only MAC algorithm on the list
and configured with specific parameters (detailed in KMAC128 for BLS)
For example, to compute a SHA3256 digest:
_10let data: [UInt8] = [1, 2, 3]_10let digest = HashAlgorithm.SHA3_256.hash(data)
Hashing with a domain tagâ€‹
hashWithTag
hashes the input data along with an input tag.
It allows instanciating independent hashing functions customized with a domain separation tag (DST).
For most of the hashing algorithms, mixing the data with the tag is done by prefixing the data with the tag and
hashing the result.
SHA2_256
,SHA2_384
,SHA3_256
,SHA3_384
,KECCAK_256
: If the tag is nonempty, the hashed message isbytes(tag)  data
wherebytes()
is the UTF8 encoding of the input string, padded with zeros till 32 bytes. Therefore tags must not exceed 32 bytes. If the tag used is empty, no data prefix is applied, and the hashed message is simplydata
(same ashash
output).KMAC128_BLS_BLS12_381
: refer to KMAC128 for BLS for details.
KMAC128 for BLSâ€‹
KMAC128_BLS_BLS12_381
is an instance of the cSHAKEbased KMAC128.
Although this is a MAC algorithm, KMAC can be used as a hash when the key is used as a nonprivate customizer.
KMAC128_BLS_BLS12_381
is used in particular as the hashing algorithm for the BLS signature scheme on the curve BLS12381.
It is a customized instance of KMAC128 and is compatible with the hashing to curve used by BLS signatures.
It is the same hasher used by the internal Flow protocol, and can be used to verify Flow protocol signatures onchain.
To define the MAC instance, KMAC128(customizer, key, data, length)
is instanciated with the following parameters
(as referred to by the NIST SHA3 Derived Functions):
customizer
is the UTF8 encoding of"H2C"
.key
is the UTF8 encoding of"FLOWV00CS00withBLS_SIG_BLS12381G1_XOF:KMAC128_SSWU_RO_POP_"
whenhash
is used. It includes the inputtag
whenhashWithTag
is used and key becomes the UTF8 encoding of"FLOW"  tag  "V00CS00withBLS_SIG_BLS12381G1_XOF:KMAC128_SSWU_RO_POP_"
.data
is the input data to hash.length
is 1024 bytes.
Signature algorithmsâ€‹
The builtin enumeration SignatureAlgorithm
provides the set of supported signature algorithms.
_16access(all)_16enum SignatureAlgorithm: UInt8 {_16 /// ECDSA_P256 is ECDSA on the NIST P256 curve._16 access(all)_16 case ECDSA_P256 = 1_16_16 /// ECDSA_secp256k1 is ECDSA on the secp256k1 curve._16 access(all)_16 case ECDSA_secp256k1 = 2_16_16 /// BLS_BLS12_381 is BLS signature scheme on the BLS12381 curve._16 /// The scheme is setup so that signatures are in G_1 (subgroup of the curve over the prime field)_16 /// while public keys are in G_2 (subgroup of the curve over the prime field extension)._16 access(all)_16 case BLS_BLS12_381 = 3_16}
Public keysâ€‹
PublicKey
is a builtin structure that represents a cryptographic public key of a signature scheme.
_30access(all)_30struct PublicKey {_30_30 access(all)_30 let publicKey: [UInt8]_30_30 access(all)_30 let signatureAlgorithm: SignatureAlgorithm_30_30 /// Verifies a signature under the given tag, data and public key._30 /// It uses the given hash algorithm to hash the tag and data._30 access(all)_30 view fun verify(_30 signature: [UInt8],_30 signedData: [UInt8],_30 domainSeparationTag: String,_30 hashAlgorithm: HashAlgorithm_30 ): Bool_30_30 /// Verifies the proof of possession of the private key._30 /// This function is only implemented if the signature algorithm_30 /// of the public key is BLS (BLS_BLS12_381)._30 /// If called with any other signature algorithm, the program aborts_30 access(all)_30 view fun verifyPoP(_ proof: [UInt8]): Bool_30_30 // creating a PublicKey is a view operation_30 access(all)_30 view init()_30}
PublicKey
supports two methods verify
and verifyPoP
.
verifyPoP
will be covered under BLS multisignature.
Public Key constructionâ€‹
A PublicKey
can be constructed using the raw key and the signing algorithm.
_10let publicKey = PublicKey(_10 publicKey: "010203".decodeHex(),_10 signatureAlgorithm: SignatureAlgorithm.ECDSA_P256_10)
The raw key value depends on the supported signature scheme:

ECDSA_P256
andECDSA_secp256k1
: The public key is an uncompressed curve point(X,Y)
whereX
andY
are two prime field elements. The raw key is represented asbytes(X)  bytes(Y)
, where
is the concatenation operation, andbytes()
is the bytes bigendian encoding left padded by zeros to the bytelength of the field prime. The raw public key is 64bytes long. 
BLS_BLS_12_381
: The public key is a G_2 element (on the curve over the prime field extension). The encoding follows the compressed serialization defined in the IETF draftirtfcfrgpairingfriendlycurves08. A public key is 96bytes long.
Public Key validationâ€‹
A public key is validated at the time of creation. Only valid public keys can be created. The validation of the public key depends on the supported signature scheme:

ECDSA_P256
andECDSA_secp256k1
: The givenX
andY
coordinates are correctly serialized, represent valid prime field elements, and the resulting point is on the correct curve (no subgroup check needed since the cofactor of both supported curves is 1). 
BLS_BLS_12_381
: The given key is correctly serialized following the compressed serialization in IETF draftirtfcfrgpairingfriendlycurves08. The coordinates represent valid prime field extension elements. The resulting point is on the curve, and is on the correct subgroup G_2. Note that the point at infinity is accepted and yields the identity public key. Such identity key can be useful when aggregating multiple keys.
Since the validation happens only at the time of creation, public keys are immutable.
_10publicKey.signatureAlgorithm = SignatureAlgorithm.ECDSA_secp256k1 // Not allowed_10publicKey.publicKey = [] // Not allowed_10_10publicKey.publicKey[2] = 4 // No effect
Invalid public keys cannot be constructed so public keys are always valid.
Signature verificationâ€‹
A signature can be verified using the verify
function of the PublicKey
:
_15let pk = PublicKey(_15 publicKey: "96142CE0C5ECD869DC88C8960E286AF1CE1B29F329BA4964213934731E65A1DE480FD43EF123B9633F0A90434C6ACE0A98BB9A999231DB3F477F9D3623A6A4ED".decodeHex(),_15 signatureAlgorithm: SignatureAlgorithm.ECDSA_P256_15)_15_15let signature = "108EF718F153CFDC516D8040ABF2C8CC7AECF37C6F6EF357C31DFE1F7AC79C9D0145D1A2F08A48F1A2489A84C725D6A7AB3E842D9DC5F8FE8E659FFF5982310D".decodeHex()_15let message : [UInt8] = [1, 2, 3]_15_15let isValid = pk.verify(_15 signature: signature,_15 signedData: message,_15 domainSeparationTag: "",_15 hashAlgorithm: HashAlgorithm.SHA2_256_15)_15// `isValid` is false
The inputs to verify
depend on the signature scheme used:
 ECDSA (
ECDSA_P256
andECDSA_secp256k1
):signature
expects the couple(r,s)
. It is serialized asbytes(r)  bytes(s)
, where
is the concatenation operation, andbytes()
is the bytes bigendian encoding left padded by zeros to the bytelength of the curve order. The signature is 64 byteslong for both curves.signedData
is the arbitrary message to verify the signature against.domainSeparationTag
is the expected domain tag. Multiple valid tag values can be used (checkhashWithTag
for more details).hashAlgorithm
is eitherSHA2_256
,SHA3_256
orKECCAK_256
. It is the algorithm used to hash the message along with the given tag (check thehashWithTag
function for more details).
As noted in hashWithTag
for SHA2_256
, SHA3_256
and KECCAK_256
, using an empty tag
results in hashing the input data only. If a signature verification
needs to be done against data without any domain tag, this can be done by using an empty domain tag ""
.
ECDSA verification is implemented as defined in ANS X9.62 (also referred by FIPS 1864 and SEC 1, Version 2.0).
A valid signature would be generated using the expected signedData
, domainSeparationTag
and hashAlgorithm
used to verify.
 BLS (
BLS_BLS_12_381
):signature
expects a G_1 point (on the curve over the prime field). The encoding follows the compressed serialization defined in the IETF draftirtfcfrgpairingfriendlycurves08. A signature is 48bytes long.signedData
is the arbitrary message to verify the signature against.domainSeparationTag
is the expected domain tag. All tags are accepted (check KMAC128 for BLS).hashAlgorithm
only acceptsKMAC128_BLS_BLS12_381
. It is the algorithm used to hash the message along with the given tag (check KMAC128 for BLS).
BLS verification performs the necessary membership check on the signature while the membership check of the public key is performed at the creation of the PublicKey
object
and is not repeated during the signature verification. In order to prevent equivocation issues, a verification under the identity public key always returns false
.
The verificaction uses a hashtocurve algorithm to hash the signedData
into a G_1
point, following the hash_to_curve
method described in the draftirtfcfrghashtocurve14.
While KMAC128 is used as a hashtofield method resulting in two field elements, the mapping to curve is implemented using the simplified SWU.
A valid signature would be generated using the expected signedData
and domainSeparationTag
, as well the same hashing to curve process.
BLS multisignatureâ€‹
BLS signature scheme allows efficient multisignature features. Multiple signatures can be aggregated
into a single signature which can be verified against an aggregated public key. This allows authenticating
multiple signers with a single signature verification.
While BLS provides multiple aggregation techniques,
Cadence supports basic aggregation tools that cover a wide list of usecases.
These tools are defined in the builtin BLS
contract, which does not need to be imported.
Proof of Possession (PoP)â€‹
Multisignature verification in BLS requires a defense against rogue publickey attacks. Multiple ways are
available to protect BLS verification. Cadence provides the proof of possession of private key as a defense tool.
The proof of possession of private key is a BLS signature over the public key itself.
The PoP signature follows the same requirements of a BLS signature (detailed in Signature verification),
except it uses a special domain separation tag. The key expected to be used in KMAC128 is the UTF8 encoding of "BLS_POP_BLS12381G1_XOF:KMAC128_SSWU_RO_POP_"
.
The expected message to be signed by the PoP is the serialization of the BLS public key corresponding to the signing private key (serialization details).
The PoP can only be verified using the PublicKey
method verifyPoP
.
BLS signature aggregationâ€‹
_10view fun aggregateSignatures(_ signatures: [[UInt8]]): [UInt8]?
Aggregates multiple BLS signatures into one. Signatures could be generated from the same or distinct messages, they could also be the aggregation of other signatures. The order of the signatures in the slice does not matter since the aggregation is commutative. There is no subgroup membership check performed on the input signatures. If the array is empty or if decoding one of the signatures fails, the program aborts.
The output signature can be verified against an aggregated public key to authenticate multiple
signers at once. Since the verify
method accepts a single data to verify against, it is only possible to
verfiy multiple signatures of the same message.
BLS public key aggregationâ€‹
_10view fun aggregatePublicKeys(_ publicKeys: [PublicKey]): PublicKey?
Aggregates multiple BLS public keys into one.
The order of the public keys in the slice does not matter since the aggregation is commutative. The input keys are guaranteed to be in the correct subgroup since subgroup membership is checked at the key creation time. If the array is empty or any of the input keys is not a BLS key, the program aborts. Note that the identity public key is a valid input to this function and it represents the identity element of aggregation.
The output public key can be used to verify aggregated signatures to authenticate multiple
signers at once. Since the verify
method accepts a single data to verify against, it is only possible to
verfiy multiple signatures of the same message.
The identity public key is a possible output of the function, though signature verifications
against identity result in false
.
In order to prevent rogue key attacks when verifying aggregated signatures, it is important to verfiy the PoP of each individual key involved in the aggregation process.
Crypto Contractâ€‹
The builtin contract Crypto
can be used to perform cryptographic operations.
The contract can be imported using import Crypto
.
Key Listsâ€‹
The crypto contract also allows creating key lists to be used for multisignature verification. For example, to verify two signatures with equal weights for some signed data:
_50import Crypto_50_50access(all)_50fun test main() {_50 let keyList = Crypto.KeyList()_50_50 let publicKeyA = PublicKey(_50 publicKey:_50 "db04940e18ec414664ccfd31d5d2d4ece3985acb8cb17a2025b2f1673427267968e52e2bbf3599059649d4b2cce98fdb8a3048e68abf5abe3e710129e90696ca".decodeHex(),_50 signatureAlgorithm: SignatureAlgorithm.ECDSA_P256_50 )_50 keyList.add(_50 publicKeyA,_50 hashAlgorithm: HashAlgorithm.SHA3_256,_50 weight: 0.5_50 )_50_50 let publicKeyB = PublicKey(_50 publicKey:_50 "df9609ee588dd4a6f7789df8d56f03f545d4516f0c99b200d73b9a3afafc14de5d21a4fc7a2a2015719dc95c9e756cfa44f2a445151aaf42479e7120d83df956".decodeHex(),_50 signatureAlgorithm: SignatureAlgorithm.ECDSA_P256_50 )_50 keyList.add(_50 publicKeyB,_50 hashAlgorithm: HashAlgorithm.SHA3_256,_50 weight: 0.5_50 )_50_50 let signatureSet = [_50 Crypto.KeyListSignature(_50 keyIndex: 0,_50 signature:_50 "8870a8cbe6f44932ba59e0d15a706214cc4ad2538deb12c0cf718d86f32c47765462a92ce2da15d4a29eb4e2b6fa05d08c7db5d5b2a2cd8c2cb98ded73da31f6".decodeHex()_50 ),_50 Crypto.KeyListSignature(_50 keyIndex: 1,_50 signature:_50 "bbdc5591c3f937a730d4f6c0a6fde61a0a6ceaa531ccb367c3559335ab9734f4f2b9da8adbe371f1f7da913b5a3fdd96a871e04f078928ca89a83d841c72fadf".decodeHex()_50 )_50 ]_50_50 // "foo", encoded as UTF8, in hex representation_50 let signedData = "666f6f".decodeHex()_50_50 let isValid = keyList.verify(_50 signatureSet: signatureSet,_50 signedData: signedData,_50 domainSeparationTag: "FLOWV0.0user",_50 )_50}
The API of the Crypto contract related to key lists is:
_72access(all)_72struct KeyListEntry {_72_72 access(all)_72 let keyIndex: Int_72_72 access(all)_72 let publicKey: PublicKey_72_72 access(all)_72 let hashAlgorithm: HashAlgorithm_72_72 access(all)_72 let weight: UFix64_72_72 access(all)_72 let isRevoked: Bool_72_72 init(_72 keyIndex: Int,_72 publicKey: PublicKey,_72 hashAlgorithm: HashAlgorithm,_72 weight: UFix64,_72 isRevoked: Bool_72 )_72}_72_72access(all)_72struct KeyList {_72_72 init()_72_72 /// Adds a new key with the given weight_72 access(all)_72 fun add(_72 _ publicKey: PublicKey,_72 hashAlgorithm: HashAlgorithm,_72 weight: UFix64_72 )_72_72 /// Returns the key at the given index, if it exists._72 /// Revoked keys are always returned, but they have `isRevoked` field set to true_72 access(all)_72 fun get(keyIndex: Int): KeyListEntry?_72_72 /// Marks the key at the given index revoked, but does not delete it_72 access(all)_72 fun revoke(keyIndex: Int)_72_72 /// Returns true if the given signatures are valid for the given signed data_72 /// `domainSeparationTag` is used to specify a scope for each signature,_72 /// and is implemented the same way as `PublicKey`'s verify function._72 access(all)_72 fun verify(_72 signatureSet: [KeyListSignature],_72 signedData: [UInt8],_72 domainSeparationTag: String_72 ): Bool_72}_72_72access(all)_72struct KeyListSignature {_72_72 access(all)_72 let keyIndex: Int_72 _72 access(all)_72 let signature: [UInt8]_72_72 access(all)_72 init(keyIndex: Int, signature: [UInt8])_72}