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aes: Add doc.odin with an AES decryption example #3932

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aes: Add doc.odin with an AES decryption example #3932

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aes: Add doc.odin with an AES decryption example
sjakk Jul 16, 2024
3337b2c
clear writing odin
sjakk Jul 16, 2024
07ae179
fix for loop writing
sjakk Jul 16, 2024
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47 changes: 47 additions & 0 deletions core/crypto/aes/doc.odin
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/*
package aes implements the AES block cipher and some common modes.


An example of how to use 'AES-ECB instance' to decrypt binary files

```odin

package foo

import "core:fmt"
import "core:crypto/aes"
import "core:os"
import "core:bytes"

main :: proc() {
data, ok := os.read_entire_file("data.bin")
if !ok {fmt.eprintln("Error Reading File")}
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key := transmute([]u8)string("aeskey")
ctx: aes.Context_ECB

aes.init_ecb(&ctx, key)

if len(data) % aes.BLOCK_SIZE != 0 {
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fmt.eprintln("Error: Data length is not a multiple of AES block size")
return
}

// Allocate space for decrypted data
srcdata := make([]byte, len(data))

// Decrypt data in 16-byte blocks
for i in 0 ..< len(data) / aes.BLOCK_SIZE {
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I would rename srcdata to plaintext and rewrite the loop like this:

for len(data) > 0 {
   aes.decrypt_ecb(&ctx, plaintext[:aes.BLOCK_SIZE], data[:aes.BLOCK_SIZE])
   data = data[aes.BLOCK_SIZE:]
   plaintext = plaintext[aes.BLOCK_SIZE:]
}

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For some reason, when I rewrite the for loop this way, it gives me no output. I'm still studying how to write more idiomatic Odin code, and I will rewrite it properly soon.

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I didn't have an encrypted file to test with so I just wrote it here.

And I think I know why. :-) We're advancing the output slice here, which means it's 0 bytes at the end.
Will need to keep a backup of the original slice, too.

_plaintext := plaintext // Preserve original slice view of the decrypted data
for len(data) > 0 {
   aes.decrypt_ecb(&ctx, _plaintext[:aes.BLOCK_SIZE], data[:aes.BLOCK_SIZE])
   data = data[aes.BLOCK_SIZE:]
   _plaintext = _plaintext[aes.BLOCK_SIZE:] // Advance second view of the same destination memory
}

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package foo
import "core:fmt"
import "core:crypto/aes"
import "core:os"
import "core:bytes"

BLOCK_SIZE :: aes.BLOCK_SIZE

main :: proc() {
	data, ok := os.read_entire_file("data.bin")
	if !ok {
		fmt.eprintln("Error Reading File")
		return
	}

	if len(data) % aes.BLOCK_SIZE != 0 {
		fmt.eprintln("Error: Data length is not a multiple of AES block size")
		return
	}

	key := transmute([]u8)string("aeskey")
	ctx: aes.Context_ECB
	aes.init_ecb(&ctx, key)
	// Allocate space for decrypted data
	plaintext := make([]byte, len(data))

	plain := plaintext // Preserve original slice view of the decrypted data
	for len(data) > 0 {
		aes.decrypt_ecb(&ctx, plain[:BLOCK_SIZE], data[:BLOCK_SIZE])
		data  = data[BLOCK_SIZE:]  // Advance encrypted data
		plain = plain[BLOCK_SIZE:] // Advance second view of the same destination memory
	}

	// Output decrypted data
	fmt.println(plaintext)
	os.write_entire_file("rcdecrypted.bin", plaintext)
}

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Does this work?

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Yes, worked like a charm

start := i * aes.BLOCK_SIZE
end := start + aes.BLOCK_SIZE
aes.decrypt_ecb(&ctx, srcdata[start:end], data[start:end])
}

// Output decrypted data
fmt.println(srcdata)
os.write_entire_file("rcdecrypted.bin",srcdata)
}
```

*/
package aes