IDX10720: Unable to create KeyedHashAlgorithm for algorithm 'HS256', the key size must be greater than: '256' bits, key has '128' bits.

Algorithm and bit counts reflect your configuration; despite "must be greater than", a key of exactly the required size is accepted. Checked across versions: 6.8.0 and earlier accepted 128-bit HMAC keys; later 6.x (6.36.0) and 7.x/8.x reject them with IDX10720 (6.x adds a link to "UnsafeRelaxHmacKeySizeValidation"). Keys under 128 bits fail earlier with IDX10653 "The encryption algorithm 'HS256' requires a key size of at least '128' bits".

Microsoft.IdentityModel.Tokens (.NET) — ArgumentOutOfRangeException from SymmetricSignatureProvider

What it means

IdentityModel refuses to use an HMAC key shorter than the hash output of the algorithm: HS256 needs at least 256 bits (32 bytes), HS384 384, HS512 512, as RFC 7518 §3.2 requires. The key in the message has 128 bits — 16 bytes, a typical length of a hand-typed secret in `appsettings.json`. Short HMAC secrets can be brute-forced offline from a single token, which would let anyone mint tokens your API accepts.

Why it happens

• A short secret in configuration

  common

  Tutorials use values like `MySecretKey12345` or `supersecret`. Their UTF-8 bytes are far below 32.

• Upgrading Microsoft.IdentityModel past early 6.x

  common

  Releases up to at least 6.8.0 signed with 128-bit HMAC keys; current 6.x, 7.x and 8.x enforce the RFC minimum, so an upgrade (often pulled in by an ASP.NET Core upgrade) breaks applications with short secrets.

• HS512 with a key sized for HS256

  occasional

  A 32-byte key is enough for HS256 but not for HS512, which needs 64 bytes.

How to fix it

1. 1.

   Generate a random key of the right size

   Use a cryptographically random value, stored in user secrets or a vault, never committed to source control.

   # 64 random bytes, base64: valid for HS256, HS384 and HS512
   openssl rand -base64 64 | tr -d '\n'
   dotnet user-secrets set "Jwt:KeyBase64" "<the value above>"   # development; use a vault in production

2. 2.

   Decode it to bytes

   Build the key from the decoded bytes so the full entropy is used and every service derives the same key.

   var keyBytes = Convert.FromBase64String(builder.Configuration["Jwt:KeyBase64"]!);
   var signingKey = new SymmetricSecurityKey(keyBytes); // 64 bytes

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