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use 512 bit keys for HMAC-SHA256 #31

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Mar 5, 2025
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use 512 bit keys for HMAC-SHA256 #31

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6 changes: 3 additions & 3 deletions file name encryption/AES-SIV-512-B64URL.md
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Expand Up @@ -46,11 +46,11 @@ let rootDirId = kdf(secret: initialSeed, len: 32, context: "rootDirId")

## Deriving Encryption Keys

All file names are encrypted using AES-SIV, which requires a 512 bit key (which is internally split into two 256 bit AES keys). Furthermore we need a 256 bit key for HMAC computations. We use the directory-specific seed from `dir.uvf` and feed it into the [KDF](../kdf/README.md):
All file names are encrypted using AES-SIV, which requires a 512 bit key (which is internally split into two 256 bit AES keys). Furthermore we need a 512 bit key for HMAC computations. We use the directory-specific seed from `dir.uvf` and feed it into the [KDF](../kdf/README.md):

```ts
let sivKey = kdf(secret: seed, len: 64, context: "siv")
let hmacKey = kdf(secret: seed, len: 32, context: "hmac")
let hmacKey = kdf(secret: seed, len: 64, context: "hmac")
```

## Mapping Directory IDs to Paths
Expand Down Expand Up @@ -238,7 +238,7 @@ flowchart TD
kdfSiv{{"kdf(secret,64,'siv')"}}
kdfSiv --> sivKey
directorySeed -->|secret:| kdfHmac
kdfHmac{{"kdf(secret,32,'hmac')"}}
kdfHmac{{"kdf(secret,64,'hmac')"}}
kdfHmac --> hmacKey
hmacKey -->|key:| hmacSha256
hmacSha256{{hmacSha256}}
Expand Down