The ending is a win for White, albeit not a simple win.  

If it were Black's move, 1 ... Kf5 would draw>  The main line is as follows:

2 b5 Rd3!

The only move that draws.  Black's threat is Rd5, followed by Rb5(+).

3 Ka2 Rd5
4 Rb1 Ke5

Or 4... Ke6.  Black gets within the "square" of the Pawn.

5 Ka3

If 5 b6 Ra5+, 6 Kb2 Rb5+, 7 Kc2, Rxc1, 8 Kxc1, Kd6 catches the pawn .

6 Ka3 Kd6
7 Ka4

7 b6 Kc6, 8 b7 Ra5+, 9 Kb3 Rb5+, 10 Kc2 Rxc1, 11 Kxc1 Kxb7.

7 ... Kc7

Black gets in front of the Pawn.

White wins by taking d3 away from the Rook.

1 Kc2! Kf5

The Rook has to stay on the third rank, in order to keep White's King from getting too near to the Pawn.

The other possibility is:

1.... Ra3
2 b5 Ra5
3 b6 Rc5+ 
4 Kd3 Rd5 
5 Re6

The reason that 1... Kf5 would draw, if Black moved first, as now Re6 wouldn't be feasible.

5... Kf5 
6 Kc4! Rb1
7 Rh6 Ke4
8 Kc5

and White will promote.

2  b5!

This is now possible, as d3 is covered.

2... Ra3

Black tries to approach from the other side of the Pawn.

3 b6 Ra6

3... Ra8, 4 Kc3 Rb8, 5 Rb1 Ke6, 6 Kc4 Kd6, 7 Kb5 (White keeps Black out of c6) Kd7, 8 Ra7 Rb7 (other moves also lose), 9 Kc6! Rb8, 10 Rb7.  There are other lines, but White has just enough tempo to win.

4 Rb1 Ra8
5 b7 Rb8
6 Rb6!

White keeps Black off of the sixth rank.

6 .... Ke5
7 Kc3 Kd5
8 Kb4

Once again, White keeps Black away from the Pawn

8... Kd5
9 Ka5 Kd4
10 Ka6 Kc6
11 Ka7

(Edited to correct a typo and missing moves.)