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Clarify secret storage format (#1695)

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Richard van der Hoff 5 months ago committed by GitHub
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1
changelogs/client_server/newsfragments/1695.clarification
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Clarify the format of account data objects for secret storage.

135
content/client-server-api/modules/secrets.md
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@ -66,6 +66,70 @@ default key.
|------------|-----------|------------------------------------------|
| key | string | **Required.** The ID of the default key. |
###### `m.secret_storage.v1.aes-hmac-sha2`
For the purposes of allowing clients to check whether a user has correctly
entered the key, keys for use with the `m.secret_storage.v1.aes-hmac-sha2`
algorithm are stored with some additional data.
When storing a key, clients SHOULD:
1. Given the secret storage key, generate 64 bytes by performing an
HKDF with SHA-256 as the hash, a salt of 32 bytes of 0, and the empty
string as the info. The first 32 bytes are used as the AES key,
and the next 32 bytes are used as the MAC key.
2. Generate 16 random bytes, set bit 63 to 0 (in order to work around
differences in AES-CTR implementations), and use this as the AES
initialization vector (IV).
3. Encrypt a message consisting of 32 byutes of 0, using AES-CTR-256 using the
AES key and IV generated above.
4. Pass the raw encrypted data through HMAC-SHA-256 using the MAC key
generated above.
5. Encode the IV from step 2, and the MAC from step 4, using [unpadded
base64](/appendices/#unpadded-base64), and store the results in the `iv`
and `mac` properties respectively in the `m.secret_storage.key.[key ID]`
account-data. (The ciphertext from step 3 is discarded after passing
through the MAC calculation.)
This process can be repeated by a client checking if the key is correct: the
MAC should match if the key is correct. Note, however, that these properties
are **optional**. If they are not present, clients must assume that the key is
valid.
Note also, that although clients SHOULD use unpadded base64 as specified above,
some existing implementations use standard [RFC4648-compliant
base64](https://datatracker.ietf.org/doc/html/rfc4648#section-4) with padding,
so clients must accept either encoding.
The structure of a `m.secret_storage.key.[key ID]` account data object for use
with this algorithm is therefore as follows:
`AesHmacSha2KeyDescription`
| Parameter | Type | Description |
|-------------|--------|------------------------------------------------------------------------------------------------------|
| name | string | Optional. The name of the key. |
| algorithm | string | **Required.** The encryption algorithm to be used for this key: `m.secret_storage.v1.aes-hmac-sha2`. |
| passphrase | object | See [deriving keys from passphrases](#deriving-keys-from-passphrases) section for a description of this property. |
| iv | string | Optional. The 16-byte initialization vector for the validation check, encoded as base64. |
| mac | string | Optional. The MAC of the result of encrypting 32 bytes of 0, encoded as base64. |
For example, it could look like:
```json
{
"name": "m.default",
"algorithm": "m.secret_storage.v1.aes-hmac-sha2",
"iv": "random+data",
"mac": "mac+of+encrypted+zeros"
}
```
##### Secret storage
Encrypted data is stored in the user's account data using the event
@ -82,7 +146,7 @@ of the data.
| Parameter | Type | Description |
|-----------|------------------|-------------|
| encrypted | {string: object} | **Required.** Map from key ID the encrypted data. The exact format for the encrypted data is dependent on the key algorithm. See the definition of `AesHmacSha2EncryptedData` in the [m.secret_storage.v1.aes-hmac-sha2](#msecret_storagev1aes-hmac-sha2) section. |
| encrypted | {string: object} | **Required.** Map from key ID the encrypted data. The exact format for the encrypted data is dependent on the key algorithm. See the definition of `AesHmacSha2EncryptedData` in the [m.secret_storage.v1.aes-hmac-sha2](#msecret_storagev1aes-hmac-sha2-1) section. |
Example:
@ -147,58 +211,41 @@ HMAC-SHA-256. The secret is encrypted as follows:
1. Given the secret storage key, generate 64 bytes by performing an
HKDF with SHA-256 as the hash, a salt of 32 bytes of 0, and with the
secret name as the info. The first 32 bytes are used as the AES key,
and the next 32 bytes are used as the MAC key
and the next 32 bytes are used as the MAC key.
2. Generate 16 random bytes, set bit 63 to 0 (in order to work around
differences in AES-CTR implementations), and use this as the AES
initialization vector. This becomes the `iv` property, encoded using
base64.
3. Encrypt the data using AES-CTR-256 using the AES key generated
above. This encrypted data, encoded using base64, becomes the
`ciphertext` property.
4. Pass the raw encrypted data (prior to base64 encoding) through
HMAC-SHA-256 using the MAC key generated above. The resulting MAC is
base64-encoded and becomes the `mac` property.
initialization vector (IV).
`AesHmacSha2EncryptedData`
3. Encrypt the data using AES-CTR-256 using the AES key and IV generated
above.
| Parameter | Type | Description
|------------|---------|------------------------------------------------------------------------|
| iv | string | **Required.** The 16-byte initialization vector, encoded as base64. |
| ciphertext | string | **Required.** The AES-CTR-encrypted data, encoded as base64. |
| mac | string | **Required.** The MAC, encoded as base64. |
4. Pass the raw encrypted data through HMAC-SHA-256 using the MAC key
generated above.
For the purposes of allowing clients to check whether a user has
correctly entered the key, clients should:
5. Encode the IV from step 2, the ciphertext from step 3, and MAC from step 4,
using [unpadded base64](/appendices/#unpadded-base64), and store them as
the `iv`, `ciphertext`, and `mac` properties respectively in the account
data object.
1. encrypt and MAC a message consisting of 32 bytes of 0 as described
above, using the empty string as the info parameter to the HKDF in
step 1.
2. store the `iv` and `mac` in the `m.secret_storage.key.[key ID]`
account-data.
**Note**: some existing implementations encode these properties using
standard [RFC4648-compliant
base64](https://datatracker.ietf.org/doc/html/rfc4648#section-4) with
padding, so clients must accept either encoding.
`AesHmacSha2KeyDescription`
The structure of the `encrypted` property of an account data object encrypted
with this algorithm is therefore as follows:
| Parameter | Type | Description |
|-------------|--------|-----------------------------------------------------------------------------------------------------------------------------------|
| name | string | Optional. The name of the key. |
| algorithm | string | **Required.** The encryption algorithm to be used for this key. Currently, only `m.secret_storage.v1.aes-hmac-sha2` is supported. |
| passphrase | object | See [deriving keys from passphrases](#deriving-keys-from-passphrases) section for a description of this property. |
| iv | string | The 16-byte initialization vector, encoded as base64. |
| mac | string | The MAC of the result of encrypting 32 bytes of 0, encoded as base64. |
`AesHmacSha2EncryptedData`
For example, the `m.secret_storage.key.key_id` for a key using this
algorithm could look like:
| Parameter | Type | Description
|------------|---------|------------------------------------------------------------------------|
| iv | string | **Required.** The 16-byte initialization vector, encoded as base64. |
| ciphertext | string | **Required.** The AES-CTR-encrypted data, encoded as base64. |
| mac | string | **Required.** The MAC, encoded as base64. |
```json
{
"name": "m.default",
"algorithm": "m.secret_storage.v1.aes-hmac-sha2",
"iv": "random+data",
"mac": "mac+of+encrypted+zeros"
}
```
and data encrypted using this algorithm could look like this:
For example, data encrypted using this algorithm could look like this:
```json
{
@ -212,7 +259,7 @@ and data encrypted using this algorithm could look like this:
}
```
###### Key representation
##### Key representation
When a user is given a raw key for `m.secret_storage.v1.aes-hmac-sha2`,
it will be presented as a string constructed as follows:
@ -232,7 +279,7 @@ it will be presented as a string constructed as follows:
When decoding a raw key, the process should be reversed, with the
exception that whitespace is insignificant in the user's input.
###### Deriving keys from passphrases
##### Deriving keys from passphrases
A user may wish to use a chosen passphrase rather than a randomly
generated key. In this case, information on how to generate the key from

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