emSecure-RSA
The default version of emSecure

emSecure-RSA is the default version of emSecure, based on RSA asymmetrical encryption.

Using emSecure-RSA

All emSecure products are created to be simple but powerful, and easy to integrate. It can be used in new products and even extend existing ones as emSecure is a software solution and no additional hardware is required. The code is completely written in ANSI C and can be used platform- and controller-independent. The required key pairs can be generated with the included tool. A pass-phrase can be used to regenerate the keys. The generated keys can be exported into different formats to be stored on the application code or loaded from a key file. This allows portability and exchangeability between different platforms. Signing data, for instance firmware images, can be performed with the included tool. It is also possible to integrate the signing process directly into a production application running on any PC or even on a microcontroller. Once a signature is generated, the signed data can be verified by its signature in an embedded application or on an external application communicating with the device. Verifying data takes less than 20 ms on a STM32F746, running at 216 MHz, which is not significantly more time for a bootloader to start firmware.

Performance

emSecure-RSA aims for portability and is designed to fit speed and size requirements for different targets. The process of verifying data is technically split up into two steps:

Compute the hash of the data.
Decrypt the signature and compare the hash values.

Step 1, the computation of the hash, depends on the size of the data, whereas step 2 is dependent on the key length and takes the same time to verify any data.

Step	Performance
Compute the hash of the data
SHA-1 Hash computation	12.5 MB/sec
Decrypt the signature and compare the hash values
RSA 512 bit Signature decryption and verification	0.84 ms
RSA 1024 bit Signature decryption and verification	2.15 ms
RSA 2048 bit Signature decryption and verification	7.29 ms
RSA 512 bit Signature generation and encryption	13.95 ms
RSA 1024 bit Signature generation and encryption	63.60 ms
RSA 2048 bit Signature generation and encryption	362.75 ms

Test configuration

SEGGER Embedded Studio 3.35 using clang 5.0.1 compiler
SHA-1 algorithm configured for speed optimization
Test on a STM32F746 microcontroller, running at 216 MHz

Verifying Data

The following table shows some total values to verify different amounts of data.

Data size	Key length	Decryption and verification	Hash computation	Total time
1 kByte	512 bit	0.84 ms	0.09 ms	0.93 ms
100 kByte	512 bit	0.84 ms	7.35 ms	8.19 ms
1 kByte	1024 bit	2.15 ms	0.09 ms	2.24 ms
100 kByte	1024 bit	2.15 ms	7.35 ms	9.50 ms
1 kByte	2048 bit	7.29 ms	0.09 ms	7.38 ms
100 kByte	2048 bit	7.29 ms	7.35 ms	14.64 ms

Signing Data

The following table shows some total values to sign different amounts of data.

Data size	Key length	Decryption and verification	Hash computation	Total time
1 kByte	512 bit	13.95 ms	0.09 ms	14.04 ms
100 kByte	512 bit	13.95 ms	7.35 ms	21.30 ms
1 kByte	1024 bit	63.60 ms	0.09 ms	63.69 ms
100 kByte	1024 bit	63.60 ms	7.35 ms	70.95 ms
1 kByte	2048 bit	362.75 ms	0.09 ms	362.84 ms
100 kByte	2048 bit	362.75 ms	7.35 ms	370.10 ms

Memory Footprint

The following table lists the memory requirements of emSecure-RSA configured for operation with a 2048 bit key.

	ROM	Static RAM	Stack
Sign only	5.81 KB	0.03 KB	2.12 KB
Verify only	4.88 KB	0.03 KB	2.93 KB
Sign and verify	6.35 KB	0.03 KB	2.93 KB

Test configuration

SEGGER Embedded Studio 3.35 using GCC compiler
SHA-1 algorithm configured for size optimization
Compiled for Thumb-2 instruction set, optimized for size

Included Sign and Verify Applications

emSecure includes all basic applications needed for securing a product. The applications' source-code is included and provides an easy to use starting point for modifications and integration into other applications.

The sign and verify applications are also available for evaluation. For more information contact us at info@segger.com

Application name	Target platform	Description
emKeyGen	Windows	Generates a key pair with a given key length, either random or from a given seed.
emSign	Windows	Digitally signs a file with your private key.
emVerify	Windows	Verifies the signature of a digital asset with its public key.
emPrintKey	Windows	Exports keys and signatures into C source format, to be included into any application.

The utilities are PC applications, ready-to-use for the setup step to secure your product.

Key Generation: emKeyGen generates a public and a private key. The generation parameters can be set with command line options. The keys are saved in a common key file format and can be published and exchanged.

Exporting Keys: emPrintKey exports key and signature files into a compileable format. The output can be linked into your application, so there is no need to load them from a file at runtime. This is especially useful for embedded applications

Signing Data: emSign digitally signs the file content, usually the data to be secured, with a given (private) key file and creates a signature file.

Verifying Data: emVerify decrypts a signature file and verifies if the corresponding data file matches the signature.

Technical Background

emSecure Signing Technical Details: The emSecure-RSA signing operation starts by using a secure hash algorithm (SHA1) to generate a hash from the original data. Using the 2kBit RSA private key along with the hash, a digital signature is generated using RSA encryption.

emSecure Verification Technical Details: The emSecure-RSA verification process starts with the data one wishes to verify and the digital signature which was created from the original file. A hash file is generated for the unverified data. The public key and RSA decryption is used to generate the original hash and then compared to verify whether the data file is genuine.