# Vault 7: Projects

This publication series is about specific projects related to the Vault 7 main publication.

### Protego

# MLA -- Crypto Library

1 January, 2012

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1.7.1.5.5 BLOCK_CIPHER_OFB_Decrypt Function
Decrypts cipher text using output feedback mode.
File
block_cipher_mode_ofb.h
Syntax
BLOCK_CIPHER_ERRORS BLOCK_CIPHER_OFB_Decrypt(DRV_HANDLE handle, uint8_t * plainText,
uint8_t * cipherText, uint32_t numBytes, void * key, BLOCK_CIPHER_OFB_CONTEXT * context,
uint32_t options);
Module
OFB
Returns
Returns a member of the BLOCK_CIPHER_ERRORS enumeration:
• BLOCK_CIPHER_ERROR_NONE - no error.
• BLOCK_CIPHER_ERROR_KEY_STREAM_GEN_OUT_OF_SPACE - There was not enough room remaining in the
context->keyStream buffer to fit the key data requested by the numBlocks parameter.
Description
Decrypts cipher text using output feedback mode.
Preconditions
The OFB context must be initialized with the block cipher encrypt/decrypt functions and the block cipher algorithm's block
size. The block cipher module must be initialized, if necessary.
The initializationVector parameter in the BLOCK_CIPHER_OFB_CONTEXT structure should be initialized. The size of this
vector is the same as the block size of the cipher you are using.
Example
// ***************************************************************
// Decrypt data in OFB mode with the AES algorithm.
// ***************************************************************
// System module object variable (for initializing AES)
SYS_MODULE_OBJ sysObject;
// Drive handle variable, to describe which AES module to use
DRV_HANDLE handle;
// OFB mode context
BLOCK_CIPHER_OFB_CONTEXT context;
// Initialization vector for OFB mode
static uint8_t initialization_vector[] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f};
// Cipher text to decrypt
static uint8_t cipher_text[] = { 0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9,
0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, 0x9e, 0xb7,
0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11, 0xe5, 0xfb,
0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef,
0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17, 0xad, 0x2b,
0x41, 0x7b, 0xe6, 0x6c, 0x37, 0x10};
// The decryption key
static uint8_t AESKey128[] = {0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6, 0xab, 0xf7,
0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c};
// Structure to contain the created AES round keys
AES_ROUND_KEYS_128_BIT round_keys;
// Buffer to contain decrypted ciphertext
1.7 Library Interface MLA - Crypto Library Help Block Cipher Modes
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