Re: [openssl-users] How to get a bye or word from BIGNUM in OpenSSL 1.1?

[Salz, Rich via openssl-users]

> It would be nice if the OpenSSL devs ate their own dogfood and suffered
> that inefficient crap. I would wager that crap would change.

There is a price to be paid with opaque structures.  In most cases the benefits 
are worth it.

I'm curious why you want that.  A small PR with doc and function should be easy 
enough.  Are you using BN as an extensible byte array?
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Re: [openssl-users] How to get a bye or word from BIGNUM in OpenSSL 1.1?

[Jakob Bohm]

On 02/08/2017 09:47, Jeffrey Walton wrote:

On Wed, Aug 2, 2017 at 12:38 AM, Jakob Bohm  wrote:

On 02/08/2017 04:21, Jeffrey Walton wrote:

I'm trying to extract the low-order byte or word from a BIGNUM in
OpenSSL 1.1. We were told to use BN_bn2binpad, but its not clear to me
how to specify the location we want to extract.

For example:

  const char v[] = "ffeeddccbbaa99887766554433221100";
  BIGNUM n = BN_new();

  if (BN_hex2bn(, v))
  {
  fprintf(stderr, "failed to parse BIGNUM\n");
  exit (1);
  }

I don't see how to get the low-order word 33221100, or the second to
last byte 11.

Here' the documentation but I don't see how to do it:
https://www.openssl.org/docs/man1.1.0/crypto/BN_bn2binpad.html. Other
libraries, like Java, Botan and Crypto++ allow us to specify a potion
to extract from in cases like these.

How do we extract bytes or words from a BIGNUM?

Thanks in advance.

BN_bn2binpad (and it's friends) always give you *all* the bytes
in the number in a buffer of you own.  You can then extract the
bytes from there.

If you care mostly about the least significant bytes, using
BN_bn2lebinpad may be easier than BN_bn2binpad, as the least
significant bytes will be first, not last, in the result.

Thanks.

It would be nice if the OpenSSL devs ate their own dogfood and
suffered that inefficient crap. I would wager that crap would change.

Jeff

Reading single bytes out of a bignum is a very rare operation
(except as part of some mathematical operation that should be
part of the bignum library).

Converting an entire bignum to/from a simple "large" int-like
variable for inclusion in various protocols and data formats is
a common operation which these functions provide.

Are you sure you are not trying to do something already provided
more directly via your "read a byte" operation?

Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
This public discussion message is non-binding and may contain errors.
WiseMo - Remote Service Management for PCs, Phones and Embedded

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Re: [openssl-users] How to get a bye or word from BIGNUM in OpenSSL 1.1?

[Jeffrey Walton]

On Wed, Aug 2, 2017 at 12:38 AM, Jakob Bohm  wrote:
> On 02/08/2017 04:21, Jeffrey Walton wrote:
>>
>> I'm trying to extract the low-order byte or word from a BIGNUM in
>> OpenSSL 1.1. We were told to use BN_bn2binpad, but its not clear to me
>> how to specify the location we want to extract.
>>
>> For example:
>>
>>  const char v[] = "ffeeddccbbaa99887766554433221100";
>>  BIGNUM n = BN_new();
>>
>>  if (BN_hex2bn(, v))
>>  {
>>  fprintf(stderr, "failed to parse BIGNUM\n");
>>  exit (1);
>>  }
>>
>> I don't see how to get the low-order word 33221100, or the second to
>> last byte 11.
>>
>> Here' the documentation but I don't see how to do it:
>> https://www.openssl.org/docs/man1.1.0/crypto/BN_bn2binpad.html. Other
>> libraries, like Java, Botan and Crypto++ allow us to specify a potion
>> to extract from in cases like these.
>>
>> How do we extract bytes or words from a BIGNUM?
>>
>> Thanks in advance.
>
> BN_bn2binpad (and it's friends) always give you *all* the bytes
> in the number in a buffer of you own.  You can then extract the
> bytes from there.
>
> If you care mostly about the least significant bytes, using
> BN_bn2lebinpad may be easier than BN_bn2binpad, as the least
> significant bytes will be first, not last, in the result.

Thanks.

It would be nice if the OpenSSL devs ate their own dogfood and
suffered that inefficient crap. I would wager that crap would change.

Jeff
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Re: [openssl-users] How to get a bye or word from BIGNUM in OpenSSL 1.1?

[Jakob Bohm]

On 02/08/2017 04:21, Jeffrey Walton wrote:

I'm trying to extract the low-order byte or word from a BIGNUM in
OpenSSL 1.1. We were told to use BN_bn2binpad, but its not clear to me
how to specify the location we want to extract.

For example:

 const char v[] = "ffeeddccbbaa99887766554433221100";
 BIGNUM n = BN_new();

 if (BN_hex2bn(, v))
 {
 fprintf(stderr, "failed to parse BIGNUM\n");
 exit (1);
 }

I don't see how to get the low-order word 33221100, or the second to
last byte 11.

Here' the documentation but I don't see how to do it:
https://www.openssl.org/docs/man1.1.0/crypto/BN_bn2binpad.html. Other
libraries, like Java, Botan and Crypto++ allow us to specify a potion
to extract from in cases like these.

How do we extract bytes or words from a BIGNUM?

Thanks in advance.

BN_bn2binpad (and it's friends) always give you *all* the bytes
in the number in a buffer of you own.  You can then extract the
bytes from there.

If you care mostly about the least significant bytes, using
BN_bn2lebinpad may be easier than BN_bn2binpad, as the least
significant bytes will be first, not last, in the result.

Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
This public discussion message is non-binding and may contain errors.
WiseMo - Remote Service Management for PCs, Phones and Embedded

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