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+# RFC-0156: Add BLS12-381 Host Functions
+
+|                 |                                                                                             |
+| --------------- | ------------------------------------------------------------------------------------------- |
+| **Start Date**  | 2025-10-15                                                                                  |
+| **Description** | Introduce BLS12-381 host functions                                                          |
+| **Author**      | Someone Unknown                                                                             |
+
+## Summary
+
+Introduce new host functions allowing runtimes to generate BLS12-381 keys, signatures, and proofs of possession.
+
+## Credits
+
+BLS implementation and initial host functions implementation are authored by Seyed Hosseini and co-authored by Davide Galassi.
+
+## Interaction with other RFCs
+
+This RFC respects the runtime-side memory allocation strategy that will be introduced by [RFC-145](https://github.com/polkadot-fellows/RFCs/pull/145).
+
+## Motivation
+
+New functions are required to equip BEEFY with BLS signatures, which are essential for the accountable light client protocol.
+
+## Stakeholders
+
+Runtime developers who will be able to use the new signature types.
+
+## Explanation
+
+This RFC proposes introducing new host functions as follows.
+
+### ext_crypto_bls381_generate
+
+Generates a BLS12-381 key for the given key type using an optional `seed`, stores it in the keystore, and returns a corresponding public key.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_bls381_generate_version_1
+ (param $id i32) (param $seed i64) (param $out i32))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `seed` is a pointer-size ([Definition 216](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer-size)) to a SCALE-encoded Option value ([Definition 200](https://spec.polkadot.network/id-cryptography-encoding#defn-option-type)) containing a BIP-39 seed which must be valid UTF-8. The function will panic if the seed is not a valid UTF-8;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 144 bytes long, where the public key will be written.
+
+### ext_crypto_bls381_generate_proof_of_possession
+
+Generates a BLS12-381 Proof Of Possession for a given public key and owner identifier.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_bls381_generate_proof_of_possession_version_1
+ (param $id i32) (param $pub_key i32) (param $owner i64) (param $out i32) (result i64))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `pub_key` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a public key, 144 bytes long;
+* `owner` is a pointer-size ([Definition 216](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer-size)) to an opaque owner identifier;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 224 bytes long, where the proof of possession will be written.
+
+#### Result
+
+The function returns `0` on success. On error, `-1` is returned, and the output buffer should be considered uninitialized.
+
+### ext_crypto_bls381_num_public_keys
+
+Retrieves the number of BLS12-381 keys of the given type available in the keystore.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_bls381_num_public_keys_version_1
+ (param $id i32) (result i32))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid.
+
+#### Result
+
+The result represents a (possibly zero) number of keys of the given type known to the keystore.
+
+### ext_crypto_bls381_public_key
+
+Retrieves a BLS12-381 public key of a given type at a given index from the keystore.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_bls381_public_key_version_1
+ (param $id i32) (param $index i32) (param $out))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `index` is an index of the key in the keystore. If the index is out of bounds (determined by the value returned by the `ext_crypto_bls381_num_public_keys` function), the function will panic;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 144 bytes long, where the key will be written.
+
+### ext_crypto_bls381_sign
+
+Signs an input message using a given BLS12-381 key.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_bls381_sign_version_1
+ (param $id i32) (param $pub_key i32) (param $msg i64) (param $out i64) (result i64))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `pub_key` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to public key bytes (as returned by `ext_crypto_bls381_public_key` function);
+* `msg` is a pointer-size ([Definition 216](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer-size)) to a message that is to be signed;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 112 bytes long, where the signature will be written.
+
+#### Result
+
+The function returns `0` on success. On error, `-1` is returned, and the output buffer should be considered uninitialized.
+
+### ext_crypto_ecdsa_bls381_generate
+
+Generates a combination ECDSA & BLS12-381 key for a given key type using an optional `seed`, stores it in the keystore, and returns the corresponding public key.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_ecdsa_bls381_generate_version_1
+ (param $id i32) (param $seed i64) (param $out i32))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `seed` is a pointer-size ([Definition 216](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer-size)) to a SCALE-encoded Option value ([Definition 200](https://spec.polkadot.network/id-cryptography-encoding#defn-option-type)) containing a BIP-39 seed which must be valid UTF-8. The function will panic if the seed is not a valid UTF-8;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 177 bytes long, where the public key will be written.
+
+### ext_crypto_ecdsa_bls381_num_public_keys
+
+Retrieves the number of ECDSA & BLS12-381 keys of a given type available in the keystore.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_ecdsa_bls381_num_public_keys_version_1
+ (param $id i32) (result i32))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid.
+
+#### Result
+
+The result represents a (possibly zero) number of keys of the given type known to the keystore.
+
+### ext_crypto_ecdsa_bls381_public_key
+
+Retrieves an ECDSA & BLS12-381 public key of a given type at a given index from the keystore.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_ecdsa_bls381_public_key_version_1
+ (param $id i32) (param $index i32) (param $out))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `index` is an index of the key in the keystore. If the index is out of bounds (determined by the value returned by the `ext_crypto_ecdsa_bls381_num_public_keys` function), the function will panic;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 177 bytes long, where the key will be written.
+
+### ext_crypto_ecdsa_bls381_sign
+
+Signs an input message using a given ECDSA & BLS12-381 key.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_ecdsa_bls381_sign_version_1
+ (param $id i32) (param $pub_key i32) (param $msg i64) (param $out i64) (result i64))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `pub_key` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to public key bytes (as returned by `ext_crypto_ecdsa_bls381_public_key` function);
+* `msg` is a pointer-size ([Definition 216](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer-size)) to a message that is to be signed;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 177 bytes long, where the signature will be written.
+
+#### Result
+
+The function returns `0` on success. On error, `-1` is returned, and the output buffer should be considered uninitialized.
+
+### ext_crypto_ecdsa_bls381_sign_with_keccak256
+
+Hashes a message using Keccak256 and then signs it using the ECDSA algorithm. It does not affect the behavior of the BLS12-381 component. Generates a BLS12-381 signature according to the IETF standard.
+
+#### Prototype
+
+```wat
+(func $ext_crypto_ecdsa_bls381_sign_with_keccak256_version_1
+ (param $id i32) (param $pub_key i32) (param $msg i64) (param $out i64) (result i64))
+```
+
+#### Arguments
+
+* `id` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to a key type identifier ([Definition 220](https://spec.polkadot.network/chap-host-api#defn-key-type-id)). The function will panic if the identifier is invalid;
+* `pub_key` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to public key bytes (as returned by `ext_crypto_ecdsa_bls381_public_key` function);
+* `msg` is a pointer-size ([Definition 216](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer-size)) to a message that is to be signed;
+* `out` is a pointer ([Definition 215](https://spec.polkadot.network/chap-host-api#defn-runtime-pointer)) to an output buffer, 177 bytes long, where the signature will be written.
+
+#### Result
+
+The function returns `0` on success. On error, `-1` is returned, and the output buffer should be considered uninitialized.