Tcl

AES-GCM Encryption / Decryption

See more Encryption Examples

Demonstrates AES-GCM encryption. Afterwards, the encrypted bytes, the authentication tag, and the IV are concatenated into one byte array and encoded to base64. For decryption, we decode the base64, extract the IV, authentication tag, and encrypted bytes, and then perform AES-GCM decryption. Tcl

load ./chilkat.dll

set success 0

# This example assumes the Chilkat API to have been previously unlocked.
# See Global Unlock Sample for sample code.

set crypt [new_CkCrypt2]

CkCrypt2_put_CryptAlgorithm $crypt "aes"
CkCrypt2_put_CipherMode $crypt "gcm"
CkCrypt2_put_KeyLength $crypt 256

set K "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F"
set IV "000102030405060708090A0B0C0D0E0F"
set AAD "feedfacedeadbeeffeedfacedeadbeefabaddad2"
set PT "This is the text to be AES-GCM encrypted."

CkCrypt2_SetEncodedIV $crypt $IV "hex"
CkCrypt2_SetEncodedKey $crypt $K "hex"

set success [CkCrypt2_SetEncodedAad $crypt $AAD "hex"]

# Return the encrypted bytes as base64
CkCrypt2_put_EncodingMode $crypt "base64"
CkCrypt2_put_Charset $crypt "utf-8"
set cipherText [CkCrypt2_encryptStringENC $crypt $PT]
if {[CkCrypt2_get_LastMethodSuccess $crypt] != 1} then {
    puts [CkCrypt2_lastErrorText $crypt]
    delete_CkCrypt2 $crypt
    exit
}

# Get the GCM authenticated tag computed when encrypting.
set authTag [CkCrypt2_getEncodedAuthTag $crypt "base64"]

puts "Cipher Text: $cipherText"
puts "Auth Tag: $authTag"

# Let's send the IV, CipherText, and AuthTag to the decrypting party.
# We'll send them concatenated like this: [IV || Ciphertext || AuthTag]
# In base64 format.
set bdEncrypted [new_CkBinData]

CkBinData_AppendEncoded $bdEncrypted $IV "hex"
CkBinData_AppendEncoded $bdEncrypted $cipherText "base64"
CkBinData_AppendEncoded $bdEncrypted $authTag "base64"

set concatenatedGcmOutput [CkBinData_getEncoded $bdEncrypted "base64"]
puts "Concatenated GCM Output: $concatenatedGcmOutput"

# Sample output so far:

# -------------------------------------------------------------------------------------
# Now let's GCM decrypt...
# -------------------------------------------------------------------------------------

set decrypt [new_CkCrypt2]

# The values shared and agreed upon by both sides beforehand are: algorithm, cipher mode, secret key, and AAD.
# Sometimes the IV can be a value already known and agreed upon, but in this case the encryptor sends the IV to the decryptor.
CkCrypt2_put_CryptAlgorithm $decrypt "aes"
CkCrypt2_put_CipherMode $decrypt "gcm"
CkCrypt2_put_KeyLength $decrypt 256
CkCrypt2_SetEncodedKey $decrypt $K "hex"
CkCrypt2_SetEncodedAad $decrypt $AAD "hex"

set bdFromEncryptor [new_CkBinData]

CkBinData_AppendEncoded $bdFromEncryptor $concatenatedGcmOutput "base64"

set sz [CkBinData_get_NumBytes $bdFromEncryptor]

# Extract the parts.
set extractedIV [CkBinData_getEncodedChunk $bdFromEncryptor 0 16 "hex"]
set extractedCipherText [CkBinData_getEncodedChunk $bdFromEncryptor 16 [expr $sz - 32] "base64"]
set expectedAuthTag [CkBinData_getEncodedChunk $bdFromEncryptor [expr $sz - 16] 16 "base64"]

# Before GCM decrypting, we must set the authenticated tag to the value that is expected.
# The decryption will fail if the resulting authenticated tag is not equal to the expected result.
set success [CkCrypt2_SetEncodedAuthTag $decrypt $expectedAuthTag "base64"]

# Also set the IV.
CkCrypt2_SetEncodedIV $decrypt $extractedIV "hex"

# Decrypt..
CkCrypt2_put_EncodingMode $decrypt "base64"
CkCrypt2_put_Charset $decrypt "utf-8"
set decryptedText [CkCrypt2_decryptStringENC $decrypt $extractedCipherText]
if {[CkCrypt2_get_LastMethodSuccess $decrypt] != 1} then {
    # Failed.  The resultant authenticated tag did not equal the expected authentication tag.
    puts [CkCrypt2_lastErrorText $decrypt]
    delete_CkCrypt2 $crypt
    delete_CkBinData $bdEncrypted
    delete_CkCrypt2 $decrypt
    delete_CkBinData $bdFromEncryptor
    exit
}

puts "Decrypted: $decryptedText"

# Sample output:

# Cipher Text: cYspSW4GuSj0Msho4OgZZ0AwspDEpTF5Br8NlA+qT3f+g3nQo+xalmU=
# Auth Tag: z/N82vdj/ZsM0WnHNCnPPw==
# Concatenated GCM Output: AAECAwQFBgcICQoLDA0OD3GLKUluBrko9DLIaODoGWdAMLKQxKUxeQa/DZQPqk93/oN50KPsWpZlz/N82vdj/ZsM0WnHNCnPPw==
# Decrypted: This is the text to be AES-GCM encrypted.

delete_CkCrypt2 $crypt
delete_CkBinData $bdEncrypted
delete_CkCrypt2 $decrypt
delete_CkBinData $bdFromEncryptor