SPECK Decryption

Speck decryption online

Speck decryption reverses Speck encryption using the same variant, key material, and compatible mode or padding settings. The SIMON and SPECK specification defines ten variants; this tool supports each of them.

How to decrypt Speck data

1. Select text or file input. For text, choose the ciphertext encoding such as Hex or Base64.
2. Select the same Speck variant and mode used for encryption.
3. Enter the direct key and IV, or reproduce the KDF settings and secret input.
4. Match the padding, salt, hash, and KDF-specific settings such as iterations or info.
5. For GCM, supply the nonce and the same AAD when it was used during encryption. Keep Append Tag enabled when the final bytes of the input contain the authentication tag. Disable it when the tag is stored separately, then enter the detached Authentication Tag. KDF-derived GCM still requires this nonce because it is not derived from the KDF.
6. Decrypt and choose the appropriate text output encoding or download the file result.

Compatibility checklist

When another tool returns a different result, compare the Speck variant, mode, padding, direct key or KDF parameters, secret input, IV or nonce, salt, info, tag, AAD, and encodings. This tool only enables GCM for Speck128 variants because GCM requires a 128-bit block cipher.

Frequently asked questions

Why can I not recover the original plaintext?

Confirm every parameter matches the encryption side. Wrong key, padding, salt, or encoding often yields a padding error, an authentication failure, or unrelated bytes rather than recoverable plaintext.

Does Speck detect tampering?

Only GCM verifies an authentication tag. CBC, CTR, CFB, OFB, and ECB do not authenticate ciphertext by themselves.